Die attach curing method for semiconductor device

ABSTRACT

A low temperature cure adhesive material for affixing a solder mask to a die is described. The adhesive material is at least partially cured at temperatures below about 100° C. The low temperature curing lowers the thermal stresses on the adhesive, diminishes the possibility of voids being formed in the adhesive material, and increases the bond yield.

FIELD OF THE INVENTION

[0001] The present invention generally relates to semiconductor devicefabrication. More particularly, the present invention relates to thecuring of an adhesive material used in affixing solder masks tosemiconductor chips.

BACKGROUND OF THE INVENTION

[0002] Some conventional semiconductor devices include chips having asolder mask and printed or screened-on conductive traces for wirebondingto a ball grid array (BGA). Generally, the solder mask is affixed to thechip by an adhesive material. Typically, the adhesive material isapplied to the chip and allowed to cure prior to deposition of thesolder mask. Currently utilized adhesive materials cure at a temperaturein excess of 150° C.

[0003] Most solder masks are formed from a liquid photoimageablematerial. Two popular solvents used in forming liquid photoimageablesolder masks are diethylene glycol monoethyl ether acetate (DGMEA) anddipropylene glycol monoethyl ether (DGME). Often a heavy aromaticnaphtha also is used as a photoinitiator. All of these materials boil atrelatively high temperatures. Specifically, DGMEA boils at 219° C., DGMEboils at 90° C., and naphtha boils at between 80° and 220° C.

[0004] Some currently used fabrication methods cure the adhesivematerial along with the solder mask. During such methods, a cure ofabout one hour at 150° C. of the liquid photoimageable solder mask iscarried out. Such a cure serves to drive the low temperature volatilecomponents of the solder mask, i.e., from the DGMEA and/or DGME, out,leaving behind the higher temperature volatiles to outgas later when thetemperature of the device in operation reaches a sufficient outgassingtemperature. Since the cure time and temperature are insufficient tocure the adhesive material, later outgassing may induce voids in theadhesive material. Voids are capable of entrapping moisture, causing thesemiconductor package to fail an environmental test. Further, outgassingcontaminates the bond pads, resulting in a low bond yield. In addition,curing at high temperatures creates thermal stresses between theadhesive material and the die which are particularly problematic forlarge and/or thin semiconductor device packages.

[0005] There exists a need for a curing methodology which inhibits theeffects of outgassing on adhesive material, thereby reducing voiding andthe collection of moisture within the adhesive material, as well aswhich reduces thermal stress on the device package and contamination ofthe bond pads.

SUMMARY OF THE INVENTION

[0006] The present invention provides a semiconductor device having asolder mask, a die and an adhesive layer affixing the die to the soldermask. The adhesive layer is cured at a temperature below about 100° C.

[0007] The present invention also provides a semiconductor device havinga solder mask, a die, electrical contacts on the solder mask and thedie, each contact on the die being wire bonded to a respective contacton the mask, and an adhesive layer affixing the die to the solder mask.The adhesive layer is cured at a temperature between about 20° C. andabout 50° higher than a glassy temperature of the adhesive layer and thecuring temperature is below about 100° C.

[0008] The present invention further provides a semiconductor packageincluding a chip, a solder mask affixed to a die by an adhesive layerwhich is cured at a temperature below about 100° C., the die beingelectrically connected to the chip, and a mold encapsulating the chip,solder mask and die.

[0009] The present invention further provides a method of forming asemiconductor device. The method includes the steps of affixing a soldermask to a semiconductor die with an adhesive layer, and curing theadhesive layer by exposing the adhesive layer to a temperature nogreater than 100° C.

[0010] These and other advantages and features of the invention will bemore readily understood from the following detailed description of theinvention which is provided in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a cross-sectional view of a conventionally fabricatedsemiconductor chip on a printed circuit board.

[0012]FIG. 2 is a cross-sectional view of a semiconductor chip on aprinted circuit board fabricated in accordance with an embodiment of theinvention.

[0013]FIG. 3 is a graph of the change in the modulus of elasticity of anadhesive material over a temperature range.

[0014]FIG. 4 is a graph of the change in the coefficient of thermalexpansion of an adhesive material over a temperature range.

[0015]FIG. 5 is a graph depicting the wire bond pull force of an 80° C.cured adhesive material versus a 125° C. cured adhesive material.

[0016]FIG. 6 is a graph depicting radii of curvature for variousadhesive materials at various temperatures and curing times.

[0017]FIG. 7 illustrates a method of forming a semiconductor package inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018]FIG. 1 illustrates a conventionally fabricated semiconductordevice 10, which includes a die 12 affixed to a solder mask 18 by anadhesive layer 14. The die 12 has contacts 20 in connection withcontacts 22 on the solder mask 18. Specifically, each contact 20 isconnected with a respective contact 22 through a wire bond 28. Thesolder mask 18 is affixed to a printed circuit board 40 or othersubstrate. The solder mask 18 contains DGMEA or DGME, or othermaterials. Further, a heavy aromatic naphtha may be used as aphotoinitiator.

[0019] During the high temperature die attach adhesive curing, hightemperature volatiles outgas from the solder mask 18 and/or the printedcircuit board 40, forming voids 16 in the adhesive layer 14. As notedabove, voids can trap moisture, causing the device 10 to fail anenvironmental test. Further, the outgassing may contaminate the contacts20, 22, thereby decreasing the likelihood of a good bond therebetween.

[0020] The present invention obviates the problems caused by hightemperature curing of the solder mask 18 by initially low temperaturecuring the adhesive layer. FIG. 2 shows a semiconductor device 100formed in accordance with the present invention. The die 12 is affixedto the solder mask 18 by an adhesive layer 114. The layer 114 issubjected to a low temperature cure, for example, below 100° C., for aperiod of time to sufficiently solidify the adhesive layer 114,rendering it more impervious to the effects of outgassing.

[0021] Referring now to FIGS. 3-4, there is a correlation between boththe Young's Modulus (E) and the coefficient of thermal expansion (CTE).Specifically, there is a limited temperature range at which a materialchanges from a flexible/pliable state to a solid. At that same limitedtemperature range, a material's CTE is changed. This limited temperaturerange is called the glassy temperature T_(g). The adhesive layer 14(FIG. 1) is formulated such that its cure temperature is greater than orequal to its glassy temperature T_(g), and since it cures at about 150°C., its T_(g) is equal to or less than about 150° C.

[0022] The adhesive layer 114 is formulated to cure at a temperaturebelow 100° C. A preferred formulation of the adhesive layer 114 includesone or more components which cure at or below 100° C. One such componentis a resin bismaleimide. The bismaleimide may be the sole component inthe layer 114 or it may be present in the adhesive layer 114 as acomponent. The adhesive layer 114 may include initiators which act as acatalyst to begin the curing at a lower temperature. One such initiatoris peroxide, which upon being heated to a temperature below 100° C.,releases free radicals. The free radicals start the chainpolymerization.

[0023] Bismaleimide has a glassy temperature T_(g) of between about 5°C. and about 10° C. To completely cure a resin, i.e., to fillycross-link the resin, a temperature of about 50° C. above the glassytemperature T_(g) is required. Thus, an adhesive layer 114 formed ofbismaleimide will cure at a temperature of about 70° C. A highertemperature would accelerate the curing process. Curing at temperaturesbelow 100° C. reduces the stresses between the adhesive layer 114 andthe die 12 and strengthens the adhesive layer 114 against voids 16caused by outgassing. Further, a low temperature cure reduces therelease of volatiles which can contaminate the contacts 20, 22, and thusa low temperature cure will provide a cleaner wire bonding site at thecontacts 20, 22.

[0024] One test to determine the viability of low temperature curing isto measure the wire pull force. Measuring the pull force allows one toascertain which adhesives that are cured at low temperature performsimilarly to adhesives which are cured at high temperatures. Typically,one would expect a lower force with a non-fully cured adhesive.Referring to FIG. 5, a pair of cured adhesives, B170 and D170, werestress tested. D170 is a die adhesive cured for four hours at 80° C.,and B170 is a die adhesive cured at 125° for one hour (manufacturer'ssuggested cure profile). The force required to pull a wire bond of D170free at 170° C. is approximately 6.75 grams, which measures favorably tothe 5.75 grams for the B170 adhesive. This result indicates that a lowtemperature cure (below 100° C.) does not affect the mechanical strengthof the material.

[0025] The stress of the adhesive may be determined by measuring theradius of curvature (ROC) measurement. The higher the stress of theadhesive, the lower the ROC. FIG. 6 illustrates the radius of curvature(ROC) of an adhesive material over a variety of conditions. Table 1illustrates the various conditions. TABLE 1 Reference Condition A curedat 125° for one hour. A125 cured at 125° for one hour and wire bonded at125° C. A170 cured at 125° for one hour and wire bonded at 170° C. Ccured at 80° for four hours. C125 cured at 80° C. for four hours andwire bonded at 125° C. C170 cured at 80° for four hours and wire bondedat 170° C. G1 cured at 150° C. for twenty minutes. G2 cured at 150° C.for forty minutes. G3 cured at 150° C. for one hour. G4 cured at 150° C.for two hours.

[0026] As indicated in FIG. 6, the low temperature cured adhesivematerial 114, denoted as C, C125 and C170, shows a higher radius ofcurvature than the adhesive materials which were high temperature cured.As noted above, the higher the ROC the lower the stress of the adhesive

[0027]FIG. 6 also indicates that as long as the adhesion is adequate,the curing of the adhesive layer 114 does not need to be complete. Morecuring can be accomplished at the following processes: wire bonding,encapsulation, solder reflow, and testing. It has been determined thatthe adhesive layer 114 subjected to a fifty percent cure exhibitssufficient adhesive strength to pass the package assembly process.

[0028] Referring to FIG. 7, next will be described a method forfabricating a semiconductor device package. The initial step 200 is toaffix the die 12 to the solder mask 18 with the adhesive material 114.Then, the adhesive material 114 undergoes a low temperature cure at step205. As indicated above, the low temperature cure is at a temperaturebelow 100° C. The contacts 20 are electrically connected by the wirebonds 28 to the contacts 22 at step 210. The entire assemblage isencapsulated in a mold at step 215. The molding process is typically ata high temperature, for example, greater than or equal to about 180° C.An optional post mold cure is then provided at step 220. The post moldcure is typically at about 175° for about four hours.

[0029] If the adhesive material 114 is not completely cured during thelow temperature cure step 205, it will become so during the subsequentheating steps 210, 220. High temperature curing, as noted above, mayintroduce high thermal stress. Nonetheless, the amount of thermal stressimparted to the adhesive material 114 is reduced since at least fiftypercent of the adhesive material 114 is cured at a low temperature.

[0030] The present invention provides an adhesive material which is lowtemperature cured, thus reducing thermal stresses and the formation ofvoids. The present invention further provides a method for making asemiconductor device including such an adhesive material.

[0031] While the invention has been described in detail in connectionwith the preferred embodiments known at the time, it should be readilyunderstood that the invention is not limited to such disclosedembodiments. Rather, the invention can be modified to incorporate anynumber of variations, alterations, substitutions or equivalentarrangements not heretofore described, but which are commensurate withthe spirit and scope of the invention. Accordingly, the invention is notto be seen as limited by the foregoing description, but is only limitedby the scope of the appended claims.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A semiconductor device comprising: a soldermask; a die; and an adhesive layer between said die and said soldermask, wherein said adhesive layer is at least partially cured at atemperature below about 100° C.
 2. The semiconductor device of claim 1,wherein said adhesive layer is at least fifty percent cured at atemperature below about 100° C.
 3. The semiconductor device of claim 1,wherein said adhesive layer is fully cured at a temperature below about100° C.
 4. The semiconductor device of claim 1, wherein said adhesivelayer is cured at a temperature between about 20° C. and about 50° C.higher than the glassy temperature of said adhesive layer.
 5. Thesemiconductor device of claim 4, wherein said adhesive layer is cured ata temperature below about 85° C.
 6. The semiconductor device claim 5,wherein said adhesive layer comprises a material with a glassytemperature between about 5° C. and about 20° C.
 7. The semiconductordevice of claim 6, wherein said adhesive layer comprises bismaleimide.8. The semiconductor device of claim 7, wherein said adhesive layerconsists essentially of bismaleimide.
 9. The semiconductor device ofclaim 1, wherein said adhesive comprises initiators which react at atemperature below about 100° C.
 10. The semiconductor device of claim 1,further comprising electrical contacts on said solder mask and said die,each said contact on said die being wire bonded to a respective saidcontact on said solder mask.
 11. The semiconductor device of claim 10,wherein said contacts are substantially free of contaminants from saidadhesive layer.
 12. A semiconductor device comprising: a solder mask; adie; electrical contacts on said solder mask and said die, each saidcontact on said die being wire bonded to a respective said contact onsaid mask, and an adhesive layer affixing said die to said solder mask,wherein said adhesive layer is cured at a temperature between about 20°C. and about 50° C. higher than a glassy temperature of said adhesivelayer and said curing temperature is below about 100° C.
 13. Thesemiconductor device of claim 12, wherein said adhesive layer is atleast partially cured at a temperature below about 100°.
 14. Thesemiconductor device of claim 13, wherein said adhesive layer is atleast fifty percent cured at a temperature below about 100° C.
 15. Thesemiconductor device of claim 12, wherein said adhesive layer is curedat a temperature below about 85° C.
 16. The semiconductor device ofclaim 15, wherein said adhesive layer comprises a material with a glassytemperature between about 5° C. and about 20° C.
 17. The semiconductordevice of claim 16, wherein said adhesive layer comprises bismaleimide.18. The semiconductor device of claim 17, wherein said adhesive layerconsists essentially of bismaleimide.
 19. The semiconductor device ofclaim 12, wherein said adhesive comprises initiators which react at atemperature below about 100° C.
 20. The semiconductor device of claim12, wherein said contacts remain relatively free of contaminantsreleased during a cure process.
 21. A method of making a semiconductordevice comprising: affixing a solder mask to a semiconductor die with anadhesive layer; and at least partially curing said adhesive layer byexposing said adhesive layer to a temperature no greater than 100° C.22. The method of claim 21, further comprising: electrically connectingcontacts on said die with contacts on said solder mask; attaching a chipto said solder mask; and encapsulating said die; solder mask and chipwith a mold.
 23. The method of claim 23, wherein said mold encapsulatesat a temperature of greater than about 100° C.
 24. The method of claim23, wherein said mold encapsulates at a temperature of about 180° C. 25.The method of claim 23, further comprising curing said mold.
 26. Themethod of claim 25, wherein said mold curing is at about 175° C.
 27. Themethod of claim 21, wherein said adhesive layer is partially cured at atemperature below about 100°.
 28. The method of claim 27, wherein saidadhesive layer is at least fifty percent cured at a temperature belowabout 100° C.
 29. The method of claim 21, wherein said adhesive layer iscured at a temperature between about 20° C. and about 50° C. higher thanglassy temperature of said adhesive layer.
 30. The method of claim 29,wherein said adhesive layer is cured at a temperature below about 85° C.